为什么在for循环中的所有其他同步代码之后，仍会运行setTimeout延迟为0的延迟？ [英] Why does a setTimeout delay of 0 still run after all other synchronous code in a for loop?

问题描述

我知道已经讨论过该问题的版本，并且我认为这是独一无二的。为什么延迟为0，仍会导致以下行为。

I've know versions of this question has been discussed, and I think this is unique. Why does a delay of 0, still causes the below behavior.

for(var i = 0; i <3; i ++) {
  console.log(i, 'started');
  setTimeout(()=> {
    console.log(i);
  },0)
  console.log(i, 'done');
}

console.log('loop over');

   // 0 started
   // 0 done
   // 1 started
   // 1 done
   // 2 started
   // 2 done
   // loop over
   // 3 3 3 

这里是到目前为止，我认为的内容：

Here is what I think I know so far:

从MDN引用有关堆栈上setTimeouts位置的信息：

Quoted from MDN in respect to setTimeouts position on the stack:

这是因为即使setTimeout的调用被延迟了
零，它仍被放置在队列中并计划在下一个
机会运行；不立即。当前执行的代码必须在执行队列中的函数之前完成
，因此生成的
执行顺序可能与预期的不同。

This is because even though setTimeout was called with a delay of zero, it's placed on a queue and scheduled to run at the next opportunity; not immediately. Currently-executing code must complete before functions on the queue are executed, thus the resulting execution order may not be as expected.

Am我正确地说是for循环，并且任何同步代码都在setTimeout之前放置在调用堆栈中，即使您将延迟设置为0，setTimeout也将始终仅运行for循环完成之后？否则，为什么延迟0仍会导致上述行为？

Am I correct in saying the for-loop and any synchronous code is placed on the call stack before setTimeout, AND even if you set the delay to 0, setTimeout will always run only after the for-loop has completed? Otherwise, why would a delay of 0, still result in the above behavior?

谢谢！

编辑：

在朝着正确的方向开始之后，我发现了一些视频和一个不错的小工具，向您展示了事件循环及其与示例代码的关系。这里是JS运行时模拟器：放大镜

After getting started in the right direction, I found a few videos and a nice little tool that shows you the event loop and how it relates to this example code. Here is the JS runtime simulator: loupe

推荐答案

JavaScript在浏览器和服务器上均作为事件循环运行。运行时会不断轮询事件。事件由用户操作（例如，单击DOM元素x），I / O（数据从I / O或ajax调用返回）以及计时器事件（在这种情况下）组成。 setTimeout（fn，0）只是简单地添加一个事件，该事件至少在经过0毫秒后才由事件循环处理。在处理完当前事件后，它将始终执行。

JavaScript, both in the browser and on the server, runs as an event loop. The runtime is constantly polling for events. Events consist of user actions (e.g. DOM element x was clicked), I/O (data came back from I/O or ajax call), and timer events (in this case). setTimeout(fn, 0) simply adds an event to be processed by the event loop at minimum when 0 milliseconds have elapsed. It will always execute after the current event has been processed.

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